1. Field of the Invention
This invention relates to an automatic mixer apparatus arranged to automatically turn on channels connected between a mixing circuit and aural signal sources generating aural signals and automatically regulate gains of channels in response to the number of aural signals generated.
2. Prior Art
Hitherto known are automatic mixer apparatus of the kind mentioned above, such as one described in U.S. Pat. No. 3,814,856, for example. Such an apparatus is generally arranged so that a plurality of microphones are respectively associated with channels each of which includes a gate circuit for turning on and off an aural signal provided to the channel from the associated microphone, a circuit for detecting the level of the aural signal on the channel and a comparator for comparing the level of the aural signal and a reference level. Also, there are separately provided further detection circuits for detecting the level of background noise signals from microphones for picking up noises and the background noise levels detected by the respective detection circuits are also provided as a reference signal to the comparators in the respective channels. Each of the comparators generates a control signal to the corresponding gate circuit to transfer the aural signal when the level of the aural signal is higher than that of the background noise. The aural signals conducted through the respective channels, or the gate circuits are added and amplified by an amplifier which is provided with a circuit for automatically reducing the gain of the amplifier in response to the number of channels with open gate circuits. Thus, if a person begins to speak through one of the microphones, the level of the aural signal from that microphone will become higher than that of the background noise so that the gate circuit of the channel associated with that microphone is opened to transfer the aural signal to the amplifier. Since only one speaker is speaking at that time, the gain of the amplifier is at a maximum rate. In this condition, if another person begins to speak through another microphone, the gate circuit in the channel associated therewith will be opened to transfer the aural signal of the second person to the amplifier. Thus, the number of gate circuits opened is increased and the amplifier gain will be reduced. The reason why the amplifier gain is controlled to be reduced in response to the increased number of open gate circuits is to enable a maximum voice magnification without any howling. The reason why only the channel gate circuits are opened in association with the microphones generating aural signals is to enhance the S/N ratio by operating only the corresponding channels.
In the prior automatic mixer apparatus mentioned above, since the gate circuit is controlled to be opened only when the level of the aural signal is larger than that of the background noise signal, the gate circuit is closed when the former is lower than the latter even when a person is talking. Accordingly, at the initial, intermediate and terminal portions of a speech, signal levels which may be lower than a noise signal level will be cut off, and thus an unnatural speech will be produced by the prior apparatus.
Further, in the prior apparatus, when a person begins to speak through a microphone, his voice may be picked up by other microphone positioned nearby and thus a plurality of channel gate circuits may simultaneously be opened. In such an event, the number of microphones generating aural signals increases to cause the amplifier gain to be reduced, although only one person is actually speaking, and thus articulation to be lowered. Even when unused microphones do not directly pick up the person's voice, since the voice will be amplified by the amplifier to produce a magnified sound from the speaker, the other microphone or microphones may pick up the magnified sound from the speaker to cause a reduction in amplifier gain and thus a reduction in articulation, as similar to above. Further, the prior art apparatus arranged to compare the level of the aural signal with that of background noise requires additional microphones for detecting background noises and circuits for detecting the level thereof. These make the apparatus complicated.